Eventually fixes #267 (closed). This is a work in progress preview of the work I'm doing on the refactoring of the error format, to make it consistent with the frontend. I have explored various designs, but I have settled on the following:

• We have a top-level type called FrontendError, which is opaque and captures two things: the BackendErrorCode, which is a "tag" uniquely identifying the particular error code that the server is emitting, and a specific payload which depends on the given BackendErrorCode. The relationship between a BackendErrorCode and its associated payload is bijective, i.e. there is a 1:1 mapping between the two. This makes for a slightly more verbose internals but the advantage is that we can be very precise in shaping the associated frontend error given a backend error.

Apart from this, I have now renamed the old GargError to be called BackendInternalError, and the idea is that throughout the backend server we throw internal backend errors and then at "the boundaries" (i.e. close to the application surface) we perform the conversion between a BackendInternalError and a FrontendError. We don't have to do this ourselves: this is done automatically as part of the backendErrorToFrontendError function which is currently WIP (we have a bunch of undefined there).

Last but not least, in the Main code that turns a Servant server into a WAI Application we will eventually call frontendErrorToServerError, which is responsible to generate a suitable ServerError (<- the servant type) which will render nicely and consistently on the frontend side.

The idea is that now the frontend will always have a valid BackendErrorCode tag returned in the error, and parsers can be written to process incoming JSON solely based on this tag.

What's remaining for this MR to become ready:

1. ✅ We need to fix all the undefined;
2. We need to replace all the panic and error we throw in the handlers with proper BackendInternalError;
3. Switch the main loop to call frontendErrorToServerError.

To be continued on Thursday and next week.